In U.S. Pat. No. 4,492,392 dated Jan. 8, 1985, and owned by the assignee of the present invention, there is disclosed a tank fitting comprising a three flanged polyolefin fitting wherein two of the flanges are integrally connected to a pipe adapted to extend through a tank wall, and a back-up flange is slidably mounted on the pipe within the tank and bolted to one of the integral flanges exterior of the tank, to thereby make a sealing connection with the tank wherein the back-up flange abuts the interior wall surface of the tank. A siphon drain is connected to the free end of the pipe within the tank, to thereby facilitate the draining of the tank.